This invention relates to humidification of unhumidified fuel cell reactant gases near the inlet of interdigitated reactant flow field channels by means of a humidification zone having non-interdigitated, low pressure flow-through flow paths, in which the dry incoming gases are exposed to a water transport plate and/or the anode but are not in contact with the membrane (or other reaction electrolyte).
In fuel cells employing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, it is critical that a proper water balance be maintained within the PEM electrolyte. The process not only generates water at the cathode, but also drags water along with the ions from the anode through the PEM electrolyte to the cathode. If the electrolyte is dry, the process effectiveness suffers, due to increased ionic resistance and reactant cross over, which generates heat and results in membrane degradation, reducing the life of the fuel cell power plant.
In some fuel cells, the reactant gases are humidified before being introduced to the fuel cell, whereby the membrane is always provided with adequate moisture. However, this requires humidification equipment which increases the weight, volume and complexity of the fuel cell. In the case of fuel cells operating at substantially atmospheric pressure, thereby to avoid the need for having a compressor which increases parasitic power waste, it is preferred to utilize unhumidified reactant gases. When this is done, the reactant gases do not become humidified at the very entrance of the fuel cell, but only after flowing adjacent to a water transport plate and/or membrane electrode assembly for a sufficient distance to add adequate moisture to the reactant gas streams. Cells utilizing water transport plates are shown in commonly owned copending U.S. patent application Ser. No. 09/733,133 filed Dec. 8, 2000, incorporated herein by reference. Therefore, the use of unhumidified reactant gases will result in a portion of the electrolyte being dry, with attendant increase in ionic resistance and reactant cross over and reaction which results in heating and degradation of the electrolyte membrane.
In commonly owned copending U.S. patent application Ser. No. 09/731,307 filed on Dec. 6, 2000 and entitled xe2x80x9cFuel Cell With an Electrolyte Dry-Out Barrierxe2x80x9d, incorporated herein by reference, electrolyte dry-out barriers are provided to either or both of the anode and cathode reactant flow fields. The barriers prevent the fuel-containing gas and the oxidant gas streams from contacting the electrolyte prior to being at least partially humidified. The barriers extend for a small percentage (in the range of 3%-10%) of the total length of the flow field.
In some PEM fuel cells, it is known to use interdigitated flow fields, in which the inlet flow channels are not connected to the outlet flow channels such as in U.S. Pat. No. 5,641,586. This design causes the reactant gases to be transported by forced convection into the adjacent porous electrode substrates, which is more effective than the diffusion process in cells with conventional flow-through flow fields. Interdigitated flow fields are used to advantage particularly when the fuel cell is operating on dilute reactants, such as air or reformed hydrocarbon fuel, in which the extraction of the desired reactant gas component becomes more critical.
It has been found that utilization of an electrolyte dry-out barrier of the type described in the aforementioned application in conjunction with interdigitated flow fields, to an extent sufficient to eliminate electrolyte dry-out, results in too great a fraction of the electrolyte area being removed from active participation in the fuel cell process, thereby reducing the effectiveness of the fuel cell. Increasing the area of the electrolyte dry-out barrier increases the weight, volume and cost of the fuel cell. This is particularly undesirable for fuel cells used in vehicular applications.
Objects of the invention include provision of an improved reactant humidification zone in a fuel cell employing an interdigitated flow field; provision of a humidification zone in an interdigitated flow field which does not neutralize a prohibitively large fraction of the electrochemically active area.
According to the present invention, a reactant gas humidification zone in a fuel cell employing reactant gas flow field plates having interdigitated reactant flow channels includes an entry portion having flow-through flow channels coextensive with an electrolyte dry-out barrier, thereby significantly reducing the amount of electrolyte that must be protected from the unhumidified entry reactant gases.
According further to the invention, flow channels in the humidification zone are more narrow and more numerous than the flow field channels, thereby to provide additional oxidant contact area.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.